1. Field of the Invention
The present invention relates to a motor driving integrated circuit.
2. Description of the Related Art
A modern electronic device often has a plurality of motors in one device. For example, a digital camera has a plurality of motors in order to drive a zoom mechanism, a focus mechanism, an aperture mechanism, a shutter unit, and the like. Therefore, a plurality of motors have been generally driven by the use of one motor driving integrated circuit (for example, see Japanese Patent Laid-Open Publication No. 2004-104940).
The driving voltages of motors controlled by the use of such a motor driving integrated circuit are often not identical among all the motors but different per motor. In this manner, in a motor driving integrated circuit for driving a plurality of motors which driving voltages are different from each other, it has been often seen that a plurality of driving voltages different per each of the motors are not applied to the motor driving integrated circuit but only the highest driving voltage of all the driving voltages is applied. A motor that dose not necessitate a high driving voltage applied from an external power source has generally been used with driving voltage lowered by constant voltage control, PWM (Pulse Width Modulation) control, or the like in a motor driving integrated circuit (for example, see Japanese Patent Laid-Open Publication No. 2005-287186).
FIG. 11 is a diagram showing a general structural example of a motor driving integrated circuit that can lower motor driving voltage by constant voltage control. A motor driving integrated circuit 100 is composed of p-channel MOSFETs 110 and 111, n-channel MOSFETs 112 and 113, a voltage reference circuit 114, resistors 115 and 116, a comparator 117, and a switching circuit 118. The circuit 100 controls the drive of a plurality of motors including a motor 120. The motor 120 is for example a DC motor that controls the zoom mechanism of a digital camera, and is connected between terminals OUT1 and OUT2 of the motor driving integrated circuit 100. A power supply voltage VM outputted from a power source 125 is applied to the motor driving integrated circuit 100 through a terminal VM. This power supply voltage VM is, for example, the highest voltage in the driving voltages of the plurality of motors that are driven by the motor driving integrated circuit 100, and is higher than the driving voltage of the motor 120.
The p-channel MOSFETs 110 and 111, and n-channel MOSFETs 112 and 113 configure an H bridge circuit. When the p-channel MOSFET 110 and n-channel MOSFET 113 are on, and the p-channel MOSFET 111 and n-channel MOSFET 112 are off, the motor 120 is energized in the direction from the terminal OUT1 to the terminal OUT2. Also, when the p-channel MOSFET 111 and n-channel MOSFET 112 are on, and the p-channel MOSFET 110 and n-channel MOSFET 113 are off, the motor 120 is energized in the direction from the terminal OUT2 to the terminal OUT1.
For example, when the motor 120 is to be energized in the direction from the terminal OUT1 to the terminal OUT2, the p-channel MOSFET 110 and n-channel MOSFET 113 are to turn on. Here, by turning the p-channel MOSFET 110 on, a voltage VOUT1 of the terminal OUT1 becomes substantially equal to the voltage VM applied at the terminal VM. Therefore, VOUT1 can be expressed as: VOUT1=VM. On the other hand, the voltage VOUT2 at the terminal OUT2 is controlled by the comparator 117 in such a way that the driving voltage (VOUT1−VOUT2) of the motor 120 becomes a predetermined voltage VM′ lower than the voltage VM. That is, when the motor 120 is energized in the direction from the terminal OUT1 to the terminal OUT2, the switching circuit 118 is switched to the terminal B, and then a gate voltage of the n-channel MOSFET 113 is controlled in such a way that the voltage at the junction of the resistors 115 and 116 becomes equal to a voltage outputted from the voltage reference circuit 114. Therefore, the voltage VOUT2=VOUT2−VM′.
As mentioned above, although the driving voltage of the motor 120 may become the voltage VM′ lower than the voltage VM by constant voltage control, a wasteful electric power is consumed in the motor driving integrated circuit because the voltage VOUT2 becomes a voltage obtained by lowering the voltage VM. That is, when the current that runs the motor 120 during constant voltage control is represented as I, in addition to a motor driving power of VM′×I, an electric power of (VM−VM′=VOUT2)×I also is consumed, and therefore power consumption increases.
Also, although it is possible to control power consumption of a motor driving integrated circuit by using PWM control instead of constant voltage control, it is necessary to input a pulse signal for PWM control, adjusted to have a predetermined duty ratio, from an external pulse generator to the motor driving integrated circuit. As a result, the number of the terminals of the motor driving integrated circuit increases, which also results in cost increase.
The present invention has been made in view of the problems above. The principal objectives of the present invention are to provide a motor driving integrated circuit that is able to restrain increase of power consumption and the number of terminals.